Achieving Clinical Success with BET Inhibitors As Anti-Cancer Agents
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www.nature.com/bjc REVIEW ARTICLE Achieving clinical success with BET inhibitors as anti-cancer agents Tatiana Shorstova1, William D. Foulkes2 and Michael Witcher 1 The transcriptional upregulation of oncogenes is a driving force behind the progression of many tumours. However, until a decade ago, the concept of ‘switching off’ these oncogenic pathways represented a formidable challenge. Research has revealed that members of the bromo- and extra-terminal domain (BET) motif family are key activators of oncogenic networks in a spectrum of cancers; their function depends on their recruitment to chromatin through two bromodomains (BD1 and BD2). The advent of potent inhibitors of BET proteins (BETi), which target either one or both bromodomains, represents an important step towards the goal of suppressing oncogenic networks within tumours. Here, we discuss the biology of BET proteins, advances in BETi design and highlight potential biomarkers predicting their activity. We also outline the logic of incorporating BETi into combination therapies to enhance its efficacy. We suggest that understanding mechanisms of activity, defining predictive biomarkers and identifying potent synergies represents a roadmap for clinical success using BETi. British Journal of Cancer (2021) 124:1478–1490; https://doi.org/10.1038/s41416-021-01321-0 BACKGROUND including both transcriptional activating or repressing marks leads Cancer is a heterogeneous disease that is characterised by to aberrant transcriptional outputs such as heightened expression abnormal cell proliferation and a range of molecular defects of oncogenes. One classical example of this is the accumulation of acquired during tumorigenesis. These renowned ‘hallmarks of transcriptionally activating lysine acetylation at enhancer regions cancer’ include sustained proliferative signalling, insensitivity to of oncogenes such as c-MYC.6 The acetylation of multiple lysine growth-suppressive signals, resistance to apoptosis, replicative residues within the N-terminal tail of core histones, mediated by immortality, angiogenesis, the capacity to invade and metastasise, histone acetyltransferases (HATs), can be recognised by proteins dysregulation of energy metabolism and the avoidance of carrying bromodomains (BRDs),7–9 which generally increase the immune detection.1 Inflammation driven by cytokine release from rate of transcription of associated genes (Fig. 1)10 through diverse tumours, genome instability and mutation are also tumour- mechanisms including the recruitment of transcriptional com- enabling characteristics. plexes and chromatin remodelling. Appropriately controlled regulation of gene expression is critical Based on the crystal structure, BRD proteins can be categorised for homoeostasis and genome stability, and an important aspect into eight families.11 One important BRD subfamily includes the of tumour biology that falls within the hallmark of genome bromo- and extra-terminal domain (BET) proteins—BRD2, BRD3, instability is transcriptional dysregulation. Dysregulated transcrip- BRD4 and BRDT. BET proteins recognise histone acetylation tional programmes trigger two major events that can promote through one of their two tandem bromodomains, BD1 or BD2, cancer: the activation of oncogenes and, conversely, the silencing and activate transcription through recruitment of the multiprotein of tumour suppressor genes. Both of these processes can mediator complex and positive transcription elongation factor b subsequently affect multiple cancer hallmarks, thereby influencing (P-TEFb), thereby enhancing transcriptional elongation.12,13 BD1 both cancer initiation and progression. To translate this biological and BD2 recognise distinct sets of acetylated histones. For concept into relevant clinical interventions, it is important to example, the BD1 pocket of BRD4 has a strong preference for identify master transcriptional regulators that drive these diverse combinations of acetylation marks on histone 4, but shows a oncogenic networks in order to pinpoint nodes for therapeutic weaker affinity for acetylated lysine residues on histone 3.11 intervention. Evidence suggests that multiple BET-family members might be Chromatin modifications, such as the post-translational mod- required for the rapid induction of target genes,14 which suggests ification of histones and DNA methylation, establish a connection that the members might have non-overlapping functions or between repressive or permissive chromatin structure and perhaps work together within a complex. transcriptional outputs.2,3 Understanding the influence of dysre- As BET proteins are important regulators of transcriptional gulated epigenetic modification on transcriptional outputs and outputs, it is not surprising that this family of proteins has using this information to uncover novel therapeutic avenues to important roles in homoeostasis and cell survival, and that their treat cancer has been an important research goal for several dysregulation can promote cancer. Consequently, attempts have decades.4,5 It is clear that dysregulation of many modifications been made to synthesise inhibitors of these proteins—BET 1Departments of Oncology and Experimental Medicine, McGill University, Lady Davis Institute and Segal Cancer Centre, Jewish General Hospital, Montreal, QC, Canada and 2Departments of Oncology and Human Genetics, McGill University, Lady Davis Institute and Segal Cancer Centre, Jewish General Hospital, Montreal, QC, Canada Correspondence: Michael Witcher ([email protected]) Received: 8 June 2020 Revised: 12 January 2021 Accepted: 11 February 2021 Published online: 15 March 2021 © The Author(s) 2021 Published by Springer Nature on behalf of Cancer Research UK Achieving clinical success with BET inhibitors as anti-cancer agents T Shorstova et al. 1479 Low acetylation Basal enhancer ‘closed’ transcription POL II Nucleosome dense Transformation from physiological condition, or during tumour progression BRD4 TF, SWI/SNF and Epigenetic changes SWI/SNF TF BRD4 recruitment ‘prime oncogene activation’ AC BRD4 AC AC AC AC AC AC AC AC POL II Nucleosome-free regions hyperacetylation of enhancers Aberrant activation of oncogenic enhancers AC TF AC AC AC AC BRD4 BRD4 Mediator BRD4 1234567890();,: p-TEFb PO4 SWI/SNF +++ AC AC POL II Enhanced transcriptional output mRNA of oncogenic networks Fig. 1 Transcriptional activation of oncogenes by BRD4. Under physiological conditions, proliferation and survival genes are transcribed at a basal rate to maintain homoeostasis. During the transformation of normal epithelium to neoplasia or, similarly, during the progression of a primary tumour to a more invasive stage, chromatin surrounding proto-oncogenes becomes enriched for histone acetylation, especially at enhancer regions. This change in chromatin programming allows nucleosome decompaction, which facilitates the recruitment of bromodomain chromatin remodellers, such as the SWI/SNF complex, that further open chromatin to allow the binding of transcription factors (TF). Acetylation also facilitates the recruitment of bromodomain-carrying coactivators, such as BRD4. BRD4 potently activates transcription through the recruitment of the Mediator complex, which connects enhancer elements with the RNA POLII complex at the promoter region of proto-oncogenes. Mediator, through association with CDK9, a component of the p-TEFb elongation complex, phosphorylates RNA POL II on serine 2 of its C-terminal domain, thereby stimulating transcriptional elongation. inhibitors (BETi)—as therapeutic agents that restore appropriately Cell cycle genes are controlled, in part, by the E2F family of regulated gene expression. We begin this review by outlining the transcription factors. BRD4 binds strongly to the regulatory involvement of BET-family members in the hallmarks of cancer, regions of E2F1 transcriptional targets to enhance their activation, especially avoiding growth suppression and resisting cell death. thereby promoting cell cycle progression.18,19 Interestingly, BRD2 Subsequently, we will introduce inhibitors of BET-family members, may associate with E2F1 and influence its targeting to regulatory elaborating on biomarkers predicting their efficacy, mechanisms regions.16,20 of resistance and enhancing their potency through the use of Oncogenic roles for BRD4 and BRD3 were first revealed from combination therapies. their propensity to translocate, forming fusion proteins with NUT (nuclear protein in testis, also known as NUTM1). The BRD4–NUT fusion (t(15;19)) is highly oncogenic and initiates the development A ROLE FOR BET PROTEINS IN CANCER of NUT-midline carcinoma (NMC), an aggressive tumour with a BET-family members influence cell cycle progression by activating very poor prognosis.21 The driving oncogenic nature of this oncogenes such as MYC, JUNB, CCND1 and CCNA1.15–17 Consistent translocation was confirmed by whole-exome sequencing, in with a role in regulating the expression of cell cycle genes, which BRD4–NUT appears as a unique genomic aberration.22 knockdown of BRD4 leads to arrest in S phase in some cell types.17 Furthermore, inhibiting BRD proteins (discussed below) reduced BET proteins may directly activate oncogene transcription through tumour cell proliferation and contributed to squamous cell recruitment to hyper-acetylated regulatory regions, as is seen is differentiation and apoptosis.23 BRD4–NUT leads to the activation the regions surrounding MYC and CCNA1 genes. Upon being of pro-survival genes such as MYC, which maintains NMC cells in recruited to chromatin,